Hi Garth,
In my previous email you saw that the BPDUs sent by the MST border switches
to MST and STP switches do contain the sender BID. Here you see the same
scenario with the non-MST switch in RPVST+ mode. F0/20 received designated
bridge ID 32768.0023.05c9.5e80. F0/21, connected to the CIST root,
received sender BID 12288.0023.05c4.bb00.
RPVST+ switch connected to two switches that are in the same MST region.
Does this scenario model your case?
Here is the root:
SW1#sh span
MST0
Spanning tree enabled protocol mstp
Root ID Priority 12288
Address 0023.05c4.bb00
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 12288 (priority 12288 sys-id-ext 0)
Address 0023.05c4.bb00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/3 Desg FWD 200000 128.5 P2p
Fa0/4 Desg FWD 200000 128.6 P2p
Fa0/5 Desg FWD 200000 128.7 P2p
Fa0/6 Desg FWD 200000 128.8 P2p
Fa0/20 Desg FWD 200000 128.22 P2p
Fa0/21 Desg FWD 200000 128.23 P2p Bound(PVST)
Fa0/22 Desg FWD 200000 128.24 P2p
Fa0/23 Desg FWD 200000 128.25 P2p
Fa0/24 Desg FWD 200000 128.26 P2p
--More--
Here is the nonroot connected to the rstp SW4:
SW2#sh span
MST0
Spanning tree enabled protocol mstp
Root ID Priority 12288
Address 0023.05c4.bb00
Cost 0
Port 25 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address 0023.05c9.5e80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/1 Desg FWD 200000 128.3 P2p
Fa0/4 Desg FWD 200000 128.6 P2p
Fa0/19 Desg FWD 200000 128.21 P2p
Fa0/20 Desg FWD 200000 128.22 P2p Bound(PVST)
Fa0/23 Root FWD 200000 128.25 P2p
Fa0/24 Altn BLK 200000 128.26 P2p
--More--
Here is the RSTP switch. F0/20 connected to SW2 and F0/21 connected to SW1
SW4#sh span
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 12288
Address 0023.05c4.bb00
Cost 19
Port 21 (FastEthernet0/19)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0023.3307.7000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/19 Root FWD 19 128.21 P2p
Fa0/20 Altn BLK 19 128.22 P2p Peer(STP)
Fa0/21 Altn BLK 19 128.23 P2p Peer(STP)
Fa0/22 Altn BLK 19 128.24 P2p
SW4#sh span int f0/20 detail
Port 22 (FastEthernet0/20) of VLAN0001 is alternate blocking
Port path cost 19, Port priority 128, Port Identifier 128.22.
Designated root has priority 12288, address *0023.05c4.bb00*
Designated bridge has priority 32768, address *0023.05c9.5e80*
Designated port id is 128.22, designated path cost 0
Timers: message age 16, forward delay 0, hold 0
Number of transitions to forwarding state: 0
Link type is point-to-point by default, Peer is STP
BPDU: sent 2, received 32
SW4#sh span int f0/21 detail
Port 23 (FastEthernet0/21) of VLAN0001 is alternate blocking
Port path cost 19, Port priority 128, Port Identifier 128.23.
Designated root has priority 12288, address *0023.05c4.bb00*
Designated bridge has priority 12288, address *0023.05c4.bb00*
Designated port id is 128.23, designated path cost 0
Timers: message age 15, forward delay 0, hold 0
Number of transitions to forwarding state: 0
Link type is point-to-point by default, Peer is STP
BPDU: sent 1, received 30
I am not able to capture bpdus in this lab, but it seems to me this is
pretty good evidence that the border switches are sending their unique BIDs
to SW4. As you say, if this was not the case, how would the tie be broken
if sending port ids are the same? The assumption in STP is that if sender
BIDs are equal, then port IDs must not be.
Best regards,
Bob Sinclair CCIE 10427 CCSI 30427
CIERS2 Online Instructor
www.tinyurl.com/ciers2online
> -----Original Message-----
> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> Garth Bryden
> Sent: Wednesday, September 08, 2010 3:54 AM
> To: bob_at_bobsinclair.net
> Cc: Cisco certification
> Subject: Re: STP Port Priority
>
> Hi Bob,
>
> Thanks for the assistance
>
> "The PVST and MST BPDUs contain two bridge IDs, the BID of the root and
> the
> BID of the sending bridge. Given equal path costs, your SW3 should
> choose
> its root based on Sending BID, regardless of sender port ID."
>
> I want to clarify what you mean here.
>
> In all versions of STP apart from MST this is definately true. Octets
> 18
> through 25 contains the senders bridge ID.
>
> In the case of MST Octets 18 through 25 contains the Regional Root ID
> which
> is the switch with the lowest cost the to CIST Root, as per the
> standard
>
> *"Octets 18 through 25 shall take the value of the CIST Regional Root
> Identifier when transmitted in RST and MST BPDUs, and the value of the
> CIST
> Bridge Identifier of the transmitting Bridge when transmitted in STP
> Configuration BPDUs. On receipt of an STP Configuration or RST BPDU,
> both
> the CIST Regional Root Identifier and the CIST Designated Bridge
> Identifier
> shall be decoded from this field. On receipt of an MST BPDU, the CIST
> Regional Root Identifier shall be decoded from this field."*
> **
> Then you have these new fields in octets 94 - 101
>
>
> *"Octets 94 through 101 convey the CIST Bridge Identifier of the
> transmitting Bridge. The 12-bit system id extension component of the
> CIST
> Bridge Identifier shall be transmitted as 0. The behavior on receipt is
> unspecified if it is non-zero"*
>
> Now in my understanding octets 94 - 101 do not exist in RSTP BPDU and I
> believe that from this, if we were interoperating with say RPVST+
> switch that had two links into two different switches in the same MST
> Region
> that the senders bridge ID would be the same, because the standard says
> that
> the value of the CIST Bridge Identifier when transmitted in STP
> Configuration BPDU's.
>
> I dont have a lab to test right now as they are all booked up, but then
> if
> we went back to my original question where "what if the ports
> connecting to
> the rpvst domain had the same port-id" what would be the tiebreaker?
>
> I Guess first off I have to see if running RPVST+ what the senders
> bridge ID
> will actually be in Cisco's implementation before getting to the second
> part.
>
> Cheers,
>
>
>
> Garth
> On Tue, Sep 7, 2010 at 10:38 PM, Bob Sinclair <bob_at_bobsinclair.net>
> wrote:
>
> > Hi Garth,
> >
> > The PVST and MST BPDUs contain two bridge IDs, the BID of the root
> and the
> > BID of the sending bridge. Given equal path costs, your SW3 should
> choose
> > its root based on Sending BID, regardless of sender port ID.
> >
> > Below you see output from a CIERS2 lab I had handy. In this case SW4
> is
> > dual-homed to an MST region. You see the root and sender bridge IDs
> are
> > the
> > same whether SW4 is PVST or MST:
> >
> > SW4 in PVST mode:
> >
> > SW4#sh span int f0/20 det
> > Port 22 (FastEthernet0/20) of VLAN0001 is blocking
> > Port path cost 19, Port priority 128, Port Identifier 128.22.
> > Designated root has priority 12288, address *0023.05c4.bb00*
> > Designated bridge has priority 32768, address *0023.05c9.5e80*
> > Designated port id is 128.22, designated path cost 0
> > Timers: message age 3, forward delay 0, hold 0
> > Number of transitions to forwarding state: 0
> > Link type is point-to-point by default
> > BPDU: sent 2, received 85
> >
> >
> > SW4#show span int f0/21 det
> > Port 23 (FastEthernet0/21) of VLAN0001 is forwarding
> > Port path cost 19, Port priority 128, Port Identifier 128.23.
> > Designated root has priority 12288, address *0023.05c4.bb00*
> > Designated bridge has priority 12288, address *0023.05c4.bb00*
> > Designated port id is 128.23, designated path cost 0
> > Timers: message age 2, forward delay 0, hold 0
> > Number of transitions to forwarding state: 1
> > Link type is point-to-point by default
> > BPDU: sent 3, received 128
> >
> >
> > SW4 in MST mode:
> >
> > SW4#show span int f0/20 det
> > Port 22 (FastEthernet0/20) of MST0 is alternate blocking
> > Port path cost 200000, Port priority 128, Port Identifier 128.22.
> > Designated root has priority 12288, address *0023.05c4.bb00*
> > Designated bridge has priority 32768, address *0023.05c9.5e80*
> > Designated port id is 128.22, designated path cost 0
> > Timers: message age 4, forward delay 0, hold 0
> > Number of transitions to forwarding state: 0
> > Link type is point-to-point by default, Boundary RSTP
> > BPDU: sent 2, received 10
> > SW4#show span int f0/21 det
> > Port 23 (FastEthernet0/21) of MST0 is root forwarding
> > Port path cost 200000, Port priority 128, Port Identifier 128.23.
> > Designated root has priority 12288, address *0023.05c4.bb00*
> > Designated bridge has priority 12288, address *0023.05c4.bb00*
> > Designated port id is 128.23, designated path cost 0
> > Timers: message age 4, forward delay 0, hold 0
> > Number of transitions to forwarding state: 1
> > Link type is point-to-point by default, Boundary RSTP
> > BPDU: sent 4, received 15
> >
> >
> > So, if I understand you question, yes: the sending bridge puts its
> own ID
> > in
> > the BPDU, but it does not replace the root BID.
> >
> > HTH,
> >
> > Bob Sinclair CCIE 10427 CCSI 30427
> > CIERS2 Online Instructor
> > www.tinyurl.com/ciers2online
> >
> >
> > > -----Original Message-----
> > > From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On
> Behalf Of
> > > Garth Bryden
> > > Sent: Tuesday, September 07, 2010 5:00 AM
> > > To: Cisco certification
> > > Subject: Re: STP Port Priority
> > >
> > > I have just built a topology based on the cabling infrastructure I
> have
> > > available to me at the moment as below.
> > >
> > > <SW1>----<SW3 >
> > > | /
> > > | /
> > > | /
> > > <SW2>/
> > >
> > > SW1 and SW2's link to SW3 is Port 16 (Both have Port ID 128.18)
> > > according to
> > > SW3
> > >
> > > SW3 link into SW 1 is Port 13
> > >
> > > SW3's link into SW2 is Port 16
> > >
> > > SW1 and SW2 are in an MSTP Region "12"
> > >
> > > SW1 is the CIST Root
> > >
> > > SW3 is running PVST+
> > >
> > > As you can see,from the below output the
> > >
> > > SW3# show spanning-tree detail
> > > VLAN0001 is executing the ieee compatible Spanning Tree protocol
> > > Bridge Identifier has priority 32768, sysid 1, address
> 0013.c419.7b80
> > > Configured hello time 2, max age 20, forward delay 15
> > > Current root has priority 0, address 0019.55bb.8b80
> > > Root port is 13 (FastEthernet0/13), cost of root path is 19
> > > Topology change flag not set, detected flag not set
> > > Number of topology changes 0 last change occurred 00:00:59 ago
> > > Times: hold 1, topology change 35, notification 2
> > > hello 2, max age 20, forward delay 15
> > > Timers: hello 0, topology change 0, notification 0, aging 300
> > >
> > > Port 13 (FastEthernet0/13) of VLAN0001 is root forwarding
> > > Port path cost 19, Port priority 128, Port Identifier 128.13.
> > > Designated root has priority 0, address 0019.55bb.8b80
> > > Designated bridge has priority 0, address *0019.55bb.8b80*
> > > *Designated port id is 128.18, designated path cost 0*
> > > Timers: message age 2, forward delay 0, hold 0
> > > Number of transitions to forwarding state: 1
> > > Link type is point-to-point by default
> > > BPDU: sent 3, received 28
> > >
> > > Port 16 (FastEthernet0/16) of VLAN0001 is alternate blocking
> > > Port path cost 19, Port priority 128, Port Identifier 128.16.
> > > Designated root has priority 0, address 0019.55bb.8b80
> > > Designated bridge has priority 32768, address *001b.d4df.bf80*
> > > *Designated port id is 128.18, designated path cost 0*
> > > Timers: message age 2, forward delay 0, hold 0
> > > Number of transitions to forwarding state: 0
> > > Link type is point-to-point by default
> > > BPDU: sent 2, received 29
> > >
> > > Does this mean that a when exiting a MSTP Region a switch will
> insert
> > > its
> > > own Bridge ID in place of the "CIST Regional Root ID"?
> > >
> > > Guess it'd make sense, there is not really any use for the CIST
> > > Regional
> > > Root bits in the BPDU outside a region?
> > >
> > > On Tue, Sep 7, 2010 at 4:41 PM, Garth Bryden <
> > > hacked.the.planet.on.28.8k.dialup_at_gmail.com> wrote:
> > >
> > > > Hello,
> > > >
> > > > I'm wondering how STP works out the Port ID part of the port
> > > priority...
> > > >
> > > > Say we have four switches
> > > >
> > > > <SW1>--------------- <SW3>
> > > > | |
> > > > | |
> > > > | |
> > > > | |
> > > > <SW 2>---------------<SW4>
> > > >
> > > >
> > > > Say SW1 + 2 are running MST Region name 12
> > > >
> > > > Say SW3 + 4 are running MST Region name 34
> > > >
> > > > The Intra Region Links on both switches are port fa0/1
> > > > The Inter Region Links on both switches are port fa0/2
> > > >
> > > > The Port Costs between the regions are the same.
> > > >
> > > > SW1 is the CIST ROOT.
> > > >
> > > > Either SW3 or SW4 will elect the Boundary Root Port towards the
> CIST
> > > Root
> > > > based on the lowest cost and become the CIST Regional Root. Now
> from
> > > what I
> > > > understand that Port Cost for CIST is only cumulative between
> > > boundary ports
> > > > and will not be adjusted within a region. So then if SW3 and SW4
> have
> > > the
> > > > same link costs there is going to be a tiebreaker, which would
> have
> > > to be
> > > > broken
> > > > based on Bridge ID.
> > > >
> > > > The Bridge ID in MST being the CIST Regional Root Bridge ID
> (Which I
> > > am
> > > > assuming going off the MSTP BPDU which has the CIST Regional Root
> ID
> > > in
> > > > place of where the Bridge ID used to be)...
> > > >
> > > > Further down, I then see that we have the "CIST Bridge ID" it
> looks
> > > as if
> > > > these a additional fields for MST. So this would likely act as
> the
> > > > tiebreaker for who becomes the CIST Regional Root in Region 34.
> > > >
> > > > Which brings me to my next question.
> > > >
> > > > If SW3 / SW4 was then running PVST+ it would not understand the
> "CIST
> > > > Bridge ID" and Region 12 would look like a single Virtual Bridge.
> > > This is
> > > > fine, except what if then my Port Priority and Port ID's are the
> > > same?
> > > >
> > > > What is the tiebreaker?
> > > >
> > > > Thanks,
> > > >
> > > > Garth
> > >
> > >
> > > Blogs and organic groups at http://www.ccie.net
> > >
> > >
> _______________________________________________________________________
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> > >
> > >
> > >
> > >
> > >
> > >
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Received on Wed Sep 08 2010 - 09:12:34 ART
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